A quick primer on how CANDUs fit into Ontario’s windy power grid – 2013 July

By: Donald Jones, P.Eng., retired nuclear industry engineer.

To show how the Ontario power grid responds to load changes (variable wind generation is negative load) on the time scale of seconds to hours and how CANDU nuclear units are integrated into the grid.

Sudden fluctuations in load on the grid (maybe from wind gusts) on a seconds to minutes timescale are taken care of by the kinetic energy of all the rotating turbine/generator masses on the grid, they would slow a little if the wind drops and inject energy into the grid to make up for the power loss from the wind. Grid frequency drops a little from 60 Hertz.

If a frequency deadband is exceeded normal turbine governor action kicks in to provide more output from the coal, gas and hydro units on the grid, primary frequency control. The present nuclear units, which operate turbine-following-reactor mode, do not contribute to this primary frequency control – see later.

If frequency offset exceeds a deadband Automatic Generation Control (AGC) will bring it back into spec. This is called secondary frequency control and is normally supplied as an IESO regulation service by select hydro units at Niagara Falls that have the appropriate load governors although coal has been used in the past. Secondary frequency control can also be done manually.

On a longer time scale the dispatching of stored water hydro and Combined Cycle Gas Turbines (CCGTs) at five minute intervals will bring the grid into the range that AGC can handle. Base load is handled by nuclear and by run-of-the-river hydro units.

On a significant load change, maybe from the intermittent characteristic of wind, stored water hydro and/or flexible coal-fired units (soon to disappear from the grid) can be brought up from low power to quickly balance the grid while any CCGTs that have been taken below their dispatchable range are brought back into their dispatchable range. Open Cycle Gas Turbines (OCGTs) are presently not needed on the Ontario grid to cater for wind, even more so now that the Independent Electricity System Operator (IESO) will have the authority to dispatch wind.

When supplying power to the Ontario grid the present CANDU units have two plant operating modes, reactor-following-turbine mode and turbine-following-reactor mode. The units were designed for base load operation with load cycling capability. Load cycling (reducing power overnight and at weekends) was originally intended to be performed with the unit in the turbine-following-reactor mode. Load following (responding to 5 minute dispatches from the IESO) could also have been done in this mode if the units had load following capability as well as in the reactor-following-turbine mode if operation were more stable in this mode – see later. Small power variations, typically +/- 2.5 percent of full power from turbine governor action to stabilize the grid when operating in reactor-following-turbine mode is called primary frequency control and is not load following.

If Ontario’s CANDUs were in reactor-following-turbine plant operating mode they could contribute to grid frequency stability. In the reactor-following-turbine mode of plant operation the steam generator pressure, which will change due to differences in reactor output and turbine-generator output, is kept at its setpoint by changing the reactor power setpoint, using the reactor regulating system, to accommodate changing turbine steam demands in response to grid conditions. Any difference between supply (generation) and demand (load) on the grid shows up as a grid frequency deviation from the nominal 60 Hz. If a unit is operating at 97.5 percent of full power it can provide +/- 2.5 percent power variation automatically by turbine governor action, to help resist the frequency change in concert with other nuclear, hydro, coal and natural gas-fired units on the grid. The more units contributing to this grid stabilization, or primary frequency control, the less the power variation will be on each unit. The designated hydro or coal units, normally hydro, supplying AGC service will then return the grid frequency to nominal by removing the frequency offset. Adjusting the turbine governor setpoint to remove the frequency offset is called secondary frequency control, or regulation, and would be performed by the IESO manually or by AGC. Fast acting AGC corrects the seconds to minutes differences in generation and load to balance the grid. The current AGC regulation service requirement from the IESO is for at least plus or minus 100 megawatts at a ramp rate of 50 megawatts per minute but this is being changed to allow other generators and even loads to supply this service. In order to keep the designated unit(s) that is on AGC service in its desired operating range, particularly during the difficult morning ramp-up and the evening ramp-down, other coal, gas and hydro units on the grid will be dispatched (load following) at 5 minute intervals to power up or power down so as to allow the unit(s) on AGC service to do its work of fine tuning the grid balance. The present CANDU units were not designed to supply AGC or manual secondary frequency control even though on at least one station provision might have been made for remote control of the turbine governor setpoint by the grid operator when in the reactor-following-turbine mode. If they were designed to supply AGC selected CANDUs would have had to operate in the reactor-following-turbine mode.

If the nuclear unit is operating in turbine-following-reactor plant operating mode it makes no contribution to grid stability. In the turbine-following-reactor mode of operation the steam generator pressure is controlled at its setpoint by operation of the turbine governor valve when the reactor power setpoint is changed for any reason, so it negates governor action. CANDUs have been operated in the reactor-following-turbine mode but in Ontario the CANDU units now operate in the turbine-following-reactor mode, preferred by operators Bruce Power and Ontario Power Generation, and at the maximum allowable power. The operators say this turbine-following-reactor mode gives more stable reactor operation and increases the probability of the unit remaining connected to the grid during major disturbances as well as generating more electricity and more income since the unit does not have to operate at a little less than its maximum output as it would have to do in the reactor-following-turbine mode.

The original Bruce B and Darlington design had the adjuster rods, used to supply a degree of flexibility, parked in the core and the intent was to allow load-cycling, not load following, using reactor power changes with no steam bypass (i.e. steam going directly to the condenser via the condenser steam discharge valves instead of going through the turbine to the condenser and performing useful work) but perhaps not to the extent now necessitated by Ontario’s many long periods of surplus base load generation. For example back in the 1980s several of the Bruce B units experienced nine months of load-cycling including deep (down to 60 percent full power, or lower) and shallow power reductions. Analytical studies based on results of in-reactor testing at the Chalk River Laboratories showed that the reactor fuel should be able to withstand daily and weekly load-cycling.

Back in the 1990s the Darlington adjuster rods were configured out of core to provide better fuel burn-up and more efficient base load operation. Presumably the same was done for Bruce B – Bruce A does not have adjusters. Bruce A and B and Darlington originally had a steam bypass system that was designed for only a limited number of turbine trip and loss of grid connection events. Bruce Power has subsequently upgraded the steam bypass system at Bruce A and B so that it can help mitigate present day surplus base load generation, including large amounts of wind, when requested by the IESO. Darlington does not allow steam bypass operation for power manoeuvring so it does not help the IESO mitigate the province’s surplus base load generation. Presently Darlington and Bruce A/B do not allow reactor power changes for power manoeuvring although a case could be made to the nuclear regulator (the Canadian Nuclear Safety Commission) to do this and improve nuclear power manoeuvring capability (reference 1) to accommodate much more wind generation on the grid before dispatching of wind generation becomes necessary.

The Enhanced CANDU 6 (EC6) proposed for new build at Darlington should be extremely flexible. It has a steam bypass system designed to discharge up to 100 percent of the steam flow directly to the condenser, bypassing the turbine, and this provides operational flexibility in support of load-following operation in conjunction with overall reactor control. This is somewhat similar to a reactor-following-turbine mode of operation except rapid changes in turbine-generator output would be made by adjusting the amount of steam bypass, much quicker than adjusting reactor power, followed up as necessary by slower changes to reactor power. This means that the EC6 could be used for AGC if required as well as for load following/cycling (reference 2). With the expected high wind content on the Ontario grid this nuclear flexibility will be needed as a replacement for coal and even more so in the future when nuclear and hydro may be the only practical energy sources for the grid (reference 3). However it still makes little economic, environmental and technical sense to manoeuvre nuclear units to accommodate unneeded wind generation.

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This entry was posted on Saturday, July 6th, 2013 at 2:44 pm and is filed under Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed.
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One Response to A quick primer on how CANDUs fit into Ontario’s windy power grid – 2013 July

[…] we don’t have that many. Most of our electricity comes from nuclear plants, which have very little flexibility to throttle up and down. Throttling a nuclear plant doesn’t really save any fuel or money, […]